Electromagnetic pickup unit



Nov. 15, 1949 R. J. ROCKWELL ELECTROMAGNETIC new? UNIT 2 Sheets-Sheet 1 Filed June 25, 1949 INVENTOR. RONALD J ROCKWELL ATTORN NOV. 15, 1949 J, ROCKWELL 2,488,504

ELECTROMAGNETIC PICKUP UNIT Filed June 25, 1949 Z SheetSQ-Sheet 2 aiii Imnlllllmll" IN V EN TOR.

Pi 5 RONALD J. ROCKWELL Patented Nov. 15, 1949 UNITED STATES PATENT OFFICE 2,488,504 ELECTROMAGNETIC PICKUP UNIT v Ronald J. Rockwell, Cincinnati, Ohio Application June 25 1949,.Serial No. 101,344

This invention relates to an electromagnetic pick-up unit for use with sound records, whereby stylus vibrations, caused by such records, are transformed to corresponding electrical manifestations, and the invention more particularly pertains to such a pick-up unit that is adapted for use with sound records of either the lateral cut or vertical cut type. The pick-up unit employs a single loop of non-magnetic, electrically conducting material, this loop functioning as the sole support for a stylus and also as the support for a magnetic member mounted for movement within a coil. When the pick-up is employed with laterally cut records, electrical currents representative of the sound modulation are induced by the movable magnetic element in the coil, the

leads of the coil being brought out to terminals. Additionally, the loop is disposed between the poles of a fixed magnet, so that it serves as a conductor element, in which current is induced, when the loop is employed with vertically cut records. Further the coil may be arranged in series circuit with the loop itself, during use with laterally cut records, so that the electrical output is furnished by both the loop and the coil, the loop moving relative to the fixed magnet and the movable magnet moving relative to the coil.

It has long been an objective in the sound reproduction art to provide a mechanically movable magnet support for an electromagnetic pick-up which is light and, therefore, requires very small forces to move it, which responds with high fidelity to the entire range of sound modulation, and which is free of the collateral noise and distortiongenerally attributable to natural vibration frequencies.

I have devised such a light weight, low-inertia magnet support embodying these desirable fea-= movement of the stylus or axial movement of the stylus, causes the loop to twist or flex. During rocking movement of the stylus the movable magnetic member moves in the coil and generates an electric current. Inasmuch as the material of which the loop is made is of an open lap-seamed 4 Claims. (c1.179100.41)

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construction, and, as the loop is made of an open lap-seamed construction, and, as the loop is long and narrow, the loop is easily twisted on its longitudinal axis and is fiexibleout of the plane of the loop, but strongly resists flexing within the plane of the loop. Such action of the loop structure is utilized so that the twisting of the loop occurs when the stylus is rocked by laterally cut records and the flexing of the loop occurs when the stylus is axially shifted by vertically cut records.

When the pick-up is employed with laterally cut records, the electrical signals induced in the coil are brought out from a pair of terminals. Additionally the loop itself, as stated above, is disposed between the poles of a fixed magnet and forms apart of a second electrical circuit, which may be connected in'circuit with the coil.

When the pick-up is employed with vertically cut records, the coil is not used. The two ends of a loop then function as one terminal and a flexible wire connection attached to the bight of the loop functions as the other terminal.

The loop is constructed of light, strong and flexible metal, forming with the stylus and movable magnetic member theonly movable mass,

which mass is of low inertia and highly responsive to stylus vibrations. The extreme ease with which the loop can be twisted and flexed, and

.its light mass, permit the stylus to follow high frequency vibrations with great fidelity. Although any light, strong, flexible, non-magnetic electrical conductor maybe used for the loop, I preferably choose an aluminum alloy including, approximately, 0.5 per cent of magnesium, 0.5

"per cent manganese, 4 per cent cooper, 94 per reproduction of high fidelity over the entire useful audible .sound range, free of disturbances caused by resonance vibrations of the part, such results beingv attributable to the novel materials and structures used in the construction of the device.

gisesoi Therefore, the principal object of my invention is to provide a novel electromagnetic sound pickup head of the type comprising a magnetic member moved relative to a coil by a stylus, in accordance with the sound modulations of a record disc.

Another object of the invention is to provide such an electromagnetic pick-up device wherein the loop, supporting the. magnetic-member. is made of tubular :material formed in along nar- 1O row U-shaped configuration.

Another object of the invention is to provides such a pick-up device wherein the tubular material of such support contains a longitudinally U-shaped formation.

Another object of the inventi-omig toprovide' such an electromagnetic pick-up device'in which: the loop, supporting the stylus and the magnetic member, is made .of a high..strength,.- flexible...20 aluminum alloy.

Another object of the invention is... to. pro-. vide such an electromagnetic pick-up in whichsuch loop is made of an electrically conducting, non-magnetic tubular material.

Another object of the invention is to provide such an electromagnetic pick-up devicein which the conductive loop consists of a single Ueshaped turn disposed in a single air gap between two poles of a permanent magnet, so that both the loopand thecoil can be used to develop the induced currents representative of the sound modulationsduring use with laterally cut records.

Another object of the invention is .to-provide such a device in which the legs ofv theloopare employed to-develop the induced currents when. sensing vertically cut records.

For a better understanding of the present invention, together with other'andzfurther objects, capabilities and advantages. thereof, reference is made to the accompanyingdrawings'wherein" there is shown an illustrative form of pick-up in". accordance with the invention.

In the drawings:

Fig. 1 is a side elevation of the novel-device, showingits disposal within a case which :isxshown in section.

Fig. 2 is a section through the device of.Fig.:1,. on the line 2.2 thereof;

Fig. 3is a perspective view of-aiportionofzthe permanent magnet asblooked .at fromtthe rear.: face, showing thedisposition of the loop between. thepole pieces thereof and the .dispositionzof acoustical damping materialv between-the pole:- pieces and the loop.

Fig. 4 is a perspectiveview of the:loop,':par.ts being broken away to show the-internal disposition of the elements.

Fig. 5is an enlarged perspective view of the ends of the loop, showing the lapped-seam construction thereof, and showing the'plastic ma terial filling the open seams.

Fig. 6 is a perspective view, with-certain parts L broken away, of the rear end of the novel-pick-up device as removed from the casing.

Fig. 7 is a side elevation of the pick-up device as removed from the casing.

Fig. 8 is abottom plan view of the device as removed from the casing.

In the drawings, the same reference numerals refer to the same parts throughoutthe several views and the sectional views are taken looking; in the direction of the arrows atthe ends of the. section lines.

0-" ofthe-legs Of-the fixed'magnet.

All of the elements of the present pick-up with the exception of four are shown and described in my cO-pending United States patent application, Serial Number 739,246, filed April 3, 1947, entitled Electromagnetic pick-up unit, now U. S. Patent No. 2,478,712, granted August 9, 1949, and assigned to the same assignee as the present invention and application. Reference is made to said co-pending patent applicationsfor-ai complete and specific description of those elements. The four elements, which are not common to both applications are: movable magnetic element or member H], a coil H and a pair of terminals [2 and I3 to which the leads of the coil are connected. The

lapped-seam disposed on the innenaspect OfnthG-l 1539 7 8 invention resides in the Provision f these additional elements and the resultant novel combinations andsub-combinations. Accordingly that portion of the description immediately following, with the exception of the discussion of these four elements, is substantially a duplication" of descriptive material contained in the aforementioned co-pending patent application:

I employ a'permanent fixed magnet 20 of ap proximately square block proportions. The mag-' net has arear face 2| (Fig.3), a front face 22' (Fig.2): and a'passagewa 23 extending fromthe rear'face to the front'face and opening down: wardly to form'the legs of a rectangular U-shapedmagnet.- I prefer to use 'a fixed magnet of ap-* proximately .4 inch in length between the front" face and the rear face,'approximately .4 inch in" height 'between the top ofthe magnet and the bottom of the legs thereof, and approximately .4 inch in width betweenthe sides thereof, with an .air'gap passagewayhaving a height of approXimately inch and' awidth of about .15 inch: For the magnet material I prefer to use an aluminum-nickel-cobalt-ir'on alloy. On the bottom'of eachleg of the magnet is soldered, or otherwise fastened,-a magnet-steel pole piece, said pieces 24 and -25 being each approximately .06 inch inthickness, each of said pole pieces being flush on' theside andrear faces of th magnet with the edges of the magnet; eachof said pole pieces extending slightly towardthe interior of :the pas-- sageway' from the-walls of the passageway so as to leave an air gap therebetween of about .093" inchi and each ofsaid pole pieces extending approximately .2 inch in front of the front face of the permanent magnet,-said'forwardly'projecting' portions ofthe pole pieces being shaded to blunt points 26'and 21 as shoWn'in'Fig. 8. In this relatively-long and narrow air gap, between'the interior edges of the pole pieces, the electricc0n= ductor loop; to-be described; is disposed. Soldered, or otherwise fastened, to therear face of the-magnet is'a brass plate 39 (Figs. 6 and 7) having a rearwardly extendingstep 3|, the bot-" tomofwhich stepis in the-plane of the bottom Brass plate 30" hasan aperture Mtherein, which is the-sizeof' thepassageway 23 opening in the magnet at its" rear end and continuous therewith.

The rearwardly-extending step 3|, of plate '30; isslightly narrower in width than the vertical portion'which is soldered to the rear magnet face 2!: This plate 30 and its "rearwardly extending ste'p- 3| act as a standard'brace for holding theelectrical loop conductor in the air gap between the pole'pieces of the magnet, and for holding a rigid conductor, used when vertical cut records are sensed, which-conductor extends through the upper part of the passageway between the legs of the magnet.

Referring especially to Figs. 3, 4, 5 and '6, I"

provide tubular conductor loop 32, which, preferably, is made of the aluminum alloy mentioned previously, from sheet material approximately .0025 inch in thickness, rolled into a lappedseamed tube having an internal diameter of approximately .01 inch, making the tubing have a minimum diameter of about .015 inch. The tube form is seen best in Fig. where the ends of the conductor loop are shown. It will be seen that the tubing is lap-seamed with an overlap of approximately 30 of the circumference, although such angle is not extremely critical. Filling the seam is plastic acoustical damping material 33 which, preferably, is a nitrocellulose-camphor product plasticized with castor oil or tricresyl phosphate and dissolved in acetone for application, so that such material, upon the drying of the acetone, assumes a tacky viscous state in which it prevents resonance vibration of the tubing without substantially interfering with the twisting and flexing action, to be described.

Referring to Fig. 4, the tube 32 is bent into a long narrow U-shaped formation, preferably, with the seam on the inner aspect of said U-shaped formation, the space between the parallel legs of the U being approximately .03 inch. As the bight formed by the tight bend of the loop has a very small radius, the tubing may be collapsed at the bend, if desired. The U-shaped formation forms an electrically conductive loop which is disposed in the air gap between the magnet pole pieces so as to very nearly, but not quite, touch the inner edges of said pole pieces when said U-shaped loop is twisted on its long axis or flexed upwardly and downwardly in a vertical manner. and the inner edges of the pole pieces and the space between the legs of the loop, is filled with a damping material, which preferably is a dimethyl-silicon-oxide polymer material containing a soap to form a viscous mass. For the purpose of decreasing the effective mass of the damping'material it may be mixed with wood pulp, that being made of balsa wood being most desirable because of its low density. This damping material 34 is shown in Fig. 3.

In the bight end of the loop is cemented a stylus 35 which, when the unit is placed in its casing presents a downwardly pointing record engaging end as shown in Figs. 1 and 2. By reference to Figs. 1, 2 and 3 it will be seen that the conductor lOop is so disposed in the air gap that its plane is horizontal, and, hence, when the stylus engages laterally cut horizontally disposed disc sound records the loop will be twisted on its long axis by the sound track, and, when the stylus is used with vertically cut horizontally disposed disc sound records, the stylus will respond V61:- tlcally and flex the loop upwardly and downwardly in the air gap. In the former circum- The space between the loop stance, the two ends of the loop form two termi- I nals to complete the usual electric circuit of the sound translating device. In the latter case the two ends of the loop are used as one terminal and the front, bight end, of the loop is used as the other terminal for completing the electric circuit of the sound translating device.

I provide pins 36 and 31 (Fig. 4) made of stiff electrically conducting material, such as steel wire, for insertion in the open ends of the tube, said pins being snugly fitted therein. The pins by their insertion into the tube have several functions. One of these functions is to act as a tuning means, whereby the natural vibration frequency of the loop may be adjusted by moving 7o magnet. As the same provision'is madeonthet the pins; or either of them in or out of the tube. Another function of these pins is to act as internal supports so that the ends of the tube may be clamped in a clamping means for supporting the loop in the air gap, as will be described, without crushing the tube ends. A third function of these pins is to act as terminals towhich electric conductors may be easily soldered when it is desired to connect the device to the electric circuit in which it is used. It will also be apparent that the internal ends of the pins which extend beyond the clamp act as cantilevers and prevent breaking of the tubes on the clamp edge.

Referring to Fig. 6, I provide a Phosphor bronze clamp 40 by which the loop conductor 32 is held securely against the bottom of the step 3 I, with a sheet of mica 4| insulating the loop from the step 3|, and a sheet of mica 42 insulating the loop from the clamp. On top of the step 3| is a sheet of mica 43 on which is placed a rigid Phosphor bronze ribbon conductor 44 which is in turn covered by a sheetof mica 45 held in place by a' Phosphor bronze plate 45. The ears 41, 48 of the clamps, are bent over to clamp the conductors tightly in place.

It will be seen, from Fig. 6, that the pins 36 and 31 are free for soldering electric terminals thereto, as is the end of conductor 44.- Conductor 44 has a forwardly extending portion which is bent angularly upwardly and then horizontally so that it passes through the passageway between the legs of the magnet near the top thereof. This conductor 44 extends slightly beyond the front face of the magnet as seen in Fig. 1 and is electrically connected to the front end of the loop by wire 5| which is made, preferably, of yielding wire about .003 inch, or less, in diameter, so that vertical or twisting movement of the loop, as it is distorted by the sound records, will not be impeded by any stiffness thereof. As seen in Fig. 1, the conductor loop 32 is given a slight downward set, when it is being secured in the clamping means, in order that when the stylus is placed on a sound record and the effective weight of the pick-up device and associated holding arms come to bear thereupon, the conductor loop will be straightened so as to assume a horizontal position in the midline point of the pole pieces between the legs of the fixed magnet. If such original downward set to the loop were not provided, the weight of the pick-up device and associated supporting arm structure would bend the loop upwardly out of the field of greatest magnetic density.

Soldered to plate 3!! (Figs. 6 and 7) is a Phosphor bronze strip 60 having a rearwardly extending portion 6| which may be used as an electric terminal for grounding purposes.

Referring to Figs. 1 and 2, I provide a casing within which the pick-up device may be mounted, said casing being made of light weight non-magnetic material. The casing has a top 10, sides H and 12, a downwardly sloping front section 13, a vertical front section 14, and a bottom closure plate 15 containing an aperture 8| through which the point of the stylus protrudes. On each of the side walls 1| and 12, in the rear part of the casing, is secured a flat plate which extends slightly inwardly of the inner wall of the casing. As shown in Fig. 1, plate 16 on wall 12 extends nearly to the top 10 leaving a space 11, which acts to provide a ledge-like slot between the top edge of the plate 16 and the top 10, which accommodates one edge of a lock-nut plate 18. The forward end of the plate abuts against the plate 30 of the,

side piece llranddslocbhut plate-l8 is made large enough-to extend .across the interior widthl of :the casing, the-said nut -p'late maybe slid in and 1 outbetween said side plates 1 and =thei top of :the:

casing; Lock-nutplate has threadedholes therein to receive locking bolts-such as bolt 19,'by whichthezbottomplate I5 is securedin place. In assembly of the device "the lock nut plate 18-i's slid-in place ethrough the open bottom of the casing the" pick-up device assembly is placed in the casing' with ithetop of the magnet against the-top of the casing. The forward upper-edge of the: magnet is positioned at the corner'between'the top and-por tion 13 of thecasing and the-rear of the magnet is held-by theside-plates described;

As the bottom plate 15 1 is put in place; the stylus rojects through' t-hehole 8 Land the bot tom plate securely holds themagnet--between=-it and: the top of the-casing. Thus the pickup unit is held in assembly.

Suitable terminal couplings --maybe providedwherebythe device is slipped onto apickup device holding arm, to complete electrical connections to an-electrical signal-translating device in the usual manner.

If desired, an insulating"sleevegsuch assleeve 80 (Fig; 1) made-of woven fabric or molded tubing, may be provided for theconductor -44 to prevent accidental contacts thereofwith the magnet.

It will be understood-that the-tubular loop con-- struction,-which I prov-ide;is unusually'strong for its dimensions; and is'especially resistant to re-- sonance vibrationinterferingwith the fidelity of reproduction-of the sensed soundrecord. Theloop is damped by reason-of the-acousticalplastie material provided in the open seamsof the tube,- and by the selective use-of the-viscous material between the sidesof the loop and -thecdges of the pole pieces,-and between-the legs of the'loop.

It is also apparent that where I-have "specified Phosphor bronze; aluminum alloy, and the like; equivalent materials havingsimilar characteris tics may be used without-in anyway departing from the spirit of the invention. Moreover, al-, though I have specified certain dimensions as de-, sirable for use with the materials-preferred,which give unusually good results, such dimensions and characteristics may be varied somewhat according to the efiects desired.

One of the most important features of this pickup device is its light weight as a whole, and in the extreme flexibility, lightness andstrength of the, moving subassembly" therein: Inertial effects damaging to therecording mediumand tending to distort the recorded sound have-by thisinVention, been reduced to a In accordance with the present invention Lprovide a movable magnetic member astationary electrical coil adapted to be placed, in inductive relation to said movable magneticmember, and an electrically conductive loop member forgsup porting both the vibratory member andsaid mQvable magnet, said conductor comprisinga, piece of open lap-seam tubing of a-U-shape form. Spe; cifically there is attached to the legs oj'the 0011-? ductive loop 32, as by a flexibleglue, a movable. magnetic member It] which may be made of the; same material as magnet 20. -This upwardly ex: tending member projects into a coil {1, composed. of insulated wire disposed Within-passageway 23. The torsional movement of the support-32, when, the pick-up is employed with laterally cut records, causes a turning movement of the-magnetic member Ill, relative to the axis of coil H, whereby resentatfve of thelisound modulation. r The in -2 sulated leads of :coil' 1 I are brought *out't'o a; pair of-terminals l2 and-l 3, those terminals Compris.

ing a pairof -fiiat str-i-ps of metal projecting for: wardly to a point nearaperture -l4.- Wh'n later ally cut records are employed theoutputelecitrical signals may be taken-= from terminals I2 and 1-3 alone, the voltage output being-relatively high-be cause of theplurality of turns of'wire oncoill-l.

1 Alternately one voltage output may be'taken'i-romterminals l2 and l f3 and another voltage output may be takenfromterminals 36 and =31; the latter being described in the aforementioned-copending patent application. 1 These voltage-outputs may be takenin series. Therefore, the in: vention also-provides a loop of the character described, which functions not only-as a stylus sup-1 port'and as-'an-electrical conductor-which has electrical current-induced in itselfby the fixed permanent magnet, but also as'a support for moving-the'movablemagnetic member relative to a fixedcoil."

During operation" withvertically cut records' themagnetic member tithe coil H, and the ter-' 'minalsl2 and 13 are :not employed," thei-latter then being simplydisimnhectedfrom the circuit.

The operation'for vertically cut records is precisely as described in-"mrsaid co-pending patent application's It will be seenthatanimprovement which-is provided bythe present-'invention'over-and above the basic-arrangementshown in "my-said copend-' ing patent applicationrresides'in theuse' of loop 32 as a supportfor *a' movable "magnet; which functions to induce-current'in a coil- I l,"having'a' large number "of terminals-"as 'comparedto the loo'p itselffwhereby'a relatively high voltage out put may be obtained" The coil H may be "secured w'ithin passageway 23 by itsown eXpanding-forcesor otherwisesuit ably secured-in place.- 'Mo-vable' magnet l0 ispreferablya flat, thinylight rectangular shaped permanent magnetr- Terminal stripslZ and t3 are secured between *micastr-ips' 4'3 and 45 and may be made of the 'samematerials-as terminal strip 44.

While therehas-"been shown" and" described what is at present consi'derewto be -the preferred embodiment of thepresentinvention, it will be obvious to those skilledin theart that various modifications and changesmay be made-therein" without departing iroimth true scope of the -invention-as defined-in the appended'claiins;

Having fully disclosed and" described my inven ,tion,'-I'- claim:

1. Vibration translatlon apparatus comprising a stylus; a stationary coil, a movable m'agnetic member for inducing-currents in said-coil; an

electrical conductor'-'for-' supporting said stylus and said magnetic members: said conductor com prising a piece-of open lap-seamed tubing "of a U-shaped loop forinr'means for 'mountingsaid loop for" torsional 'fiexure andmeans for" mounting said coil.

2. In a pick-up device'rthe' combiriation of a stylus, an electr ical-nconductor; a movable 'magnetic member=in inductive relatioir-to said conductor', a distortablessupport for said stylus and said magnetic member said support'comprising a llong U-shaped loop-of open lap-seamed tubular non-magnetic metal, a, clamp in which the legs of the p are mounted in contact-with the clampso the 'loop may 'betwisted" on its long axis; or

bent out of its plane, but which resists'distortion there are induced-in coil H electriccurrentsrep---76*in its plan'eyandmeans for mountingsaid" con--' ductor in proximity to said magnetic member so that currents are induced in said conductor.

3. In a pick-up device, the combination of field-producing means including a pair of poles arranged to form a single long narrow air gap, a

stylus, a U-shaped conductive support for said stylus, said support comprising a tube of metallic non-magnetic material having a lapped longitudinal seam on the interior of the U-shaped tube, the legs of the U being substantially parallel and slightly separated, the U formation fitting within the air gap along its length and in inductive relation to the field-producing means but without touching said poles and the free ends of the U extending slightly at one end of the gap, a non-conductive bracket holding the extending free ends of the U, whereby the support forms a loop that may be twisted and flexed in the air gap, a movable magnetic member secured to said support, a stationary coil, and means for mounting said coil so that said movable magnetic member projects into the coil, both said support and said coil having electric currents induced therein when said stylus tracks laterally modulated sound record discs.

4. An electromagnetic pick-up device cooperable with stylus vibrating types of sound records, including in combination, a long narrow U- shaped electrical conductor made of flexible tubular material and having an open lap-seam, means for holding the free ends of the U-shaped conductor insulated electrically, a, fixed magnet having opposed pole pieces forming a long narrow air gap, said holding means disposing the U- shaped conductor along said air gap and spaced slightly from but inductively related to the poles, substantially in the region of greatest magnetic density, and a stylus held in the bight end of the U-shaped conductor so that rocking the stylus twists said conductor and so that axial movement of said stylus flexes said conductor out of its normal plane, whereby to cut the lines of magnetic force, a coil fixed-secured to and insulated from said fixed magnet and a movable magnetic member projecting into said coil, said movable magnetic member being secured to said U-shaped conductor.

RONALD J. ROCKWELL.

No references cited. 

